General Axle/tranny swap questions

A "sure grip" is only mopars name for an LSD, a limited slip differential. Just as a posi, or positraction, is chevies name for an lsd. Seems like each brand had a brand-specific name for their own lsd.
An 8 3/4 has basically two types of sure grip, a cone style, and a clutch style. Clutch is stronger and rebuilable, cone is not as strong nor as "sure" in its grip.
My suggestion would be a new clutch style SG from Dr Diff if he has them back in stock. Best advice, give him a call after new years, he can help you out with advice, and parts.